libpayload/Makefile.payload: Fix CFLAGS

GCC_CFLAGS_<arch> contains only the flags that are GCC specific, iow.
flags that don't work with Clang.

CFLAGS_<arch>, OTOH, contains all flags that should be used and auto-
matically includes GCC_CFLAGS_<arch> if GCC is selected.

Change-Id: I5ec15f169d51c7a32ca86e54a98a2ce0e3b51e6d
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/62248
Reviewed-by: Thomas Heijligen <src@posteo.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/libpayload/Makefile.payload

@@ -36,7 +36,7 @@ ARCH ?=
 OBJS ?=
 CCACHE ?=
 
-CFLAGS  = $(GCC_CFLAGS_$(ARCH))
+CFLAGS  = $(CFLAGS_$(ARCH))
 CFLAGS += -Os -ffreestanding
 CFLAGS += -Wall -Wextra -Wmissing-prototypes -Wvla -Werror